The present invention generally relates to junction solar cells, and more particularly, to a radiation hardened structure for such solar cells producing an increase in both the beginning of life and end of life efficiencies for a given application.
All solar cells used in space applications suffer degradation as a result of bombardment by energetic electrons and protons trapped in the earth's magnetic field and by solar flare protons. Whereas some of the less energetic of the trapped protons may be shielded against by the cover slide, all of the other particles cause damage within the solar cell. This damage is in the form of defects in the crystal structure and has the effect of reducing the life time of minority carriers, thereby increasing the likelihood of recombination before the carriers reach the junction. Those photogenerated carriers which are not collected by the junction do not contribute to the photocurrent. This effect is most profound in cells made from highly doped substrates which would exhibit high voltage output. Therefore, current practice in the fabrication of junction solar cells is a trade-off between power output and radiation degradation. More specifically, the substrate is only modestly doped (to a resistivity between 2 and 10 .OMEGA.cm in p-Si substrates), thus sacrificing beginning of life efficiency in order to keep end of life efficiency as high as possible.